The present invention relates to a sputtering target for a magnetic recording medium for use in the deposition of a magnetic thin film of a magnetic recording medium, particularly for use in the deposition of a magnetic recording layer of a hard disk adopting the vertical magnetic recording system, and to a process for producing such a sputtering target.
In the field of magnetic recording as represented with hard disk drives, materials based on Co, Fe or Ni, which are ferromagnetic metals, are used as materials of a magnetic thin film in a magnetic recording medium. As the recording layer of hard disks adopting the vertical magnetic recording system that has been put into practical use in recent years, used is a composite material configured from a Co—Cr-based, Co—Cr—Pt-based or Co—Pt-based ferromagnetic alloy having Co as its main component and nonmagnetic inorganic particles.
A magnetic thin film of a magnetic recording medium such as a hard disk is often produced by sputtering a ferromagnetic sputtering target made from foregoing materials via sputtering in light of its high productivity. A hard disk drive that is used as an external recording device is being demanded of a higher recording density year by year, and there are strong demands for reducing the particles that are generated during sputtering pursuant to the increase in the recording density.
With a sputtering target for a magnetic recording medium made from a Co—Pt—B-based alloy or a Co—Cr—Pt—B-based alloy, since a discharge will not occur during sputtering if the leakage flux density of the sputtering target is low, the voltage during sputtering needs to be increased when the leakage flux density is low. Meanwhile, if the voltage during sputtering is increased, there is a problem in that arcing is generated or the voltage becomes unstable, and the generation of particles during sputtering increases. Thus, under normal circumstances, cold rolling is performed to artificially introduce strain and thereby increase the leakage flux density.
However, when the Co—Pt—B-based alloy or the like is subject to cold rolling, there is a problem in that the brittle B-rich phase in the alloy is subject to brittle fracture, and cracks are generated. In addition, the cracked B-rich phase becomes the source of arcing during the sputtering process and causes the generation of particles, and deteriorates the production yield. In particular, when the area of the B-rich phase is large, since the stress concentration will increase by that much, the strength will deteriorate and the generation of cracks will increase even more.
Thus, considered may be preventing the cracks in the B-rich phase. Nevertheless, conventional technologies have no recognition of this problem, and do not propose any means for solving this problem.
For example, Patent Document 1 discloses a Co—Pt—B-based target containing 1≤B≤10 (at. %) and a method for producing such a target. With this production method, hot rolling is performed at a temperature of 800 to 1100° C., and heat treatment is performed at 800 to 1100° C. for 1 hour or longer before the hot rolling process. Moreover, Patent Document 1 describes that, while hot rolling is difficult when B is contained, the generation of cracks of the ingot during hot rolling can be inhibited by controlling the temperature.
Nevertheless, Patent Document 1 does not in any way describe the problem of cracks in the B-rich phase, or the relation between the size and cracks of the B-rich phase.
Patent Document 2 discloses sputtering targets based on CoCrPt, CoCrPtTa, and CoCrPtTaZr containing B as an essential component. Patent Document 2 describes that, with this technology, the rolling properties can be improved by reducing the Cr—B-based intermetallic compound phase.
As the production method and production process, Patent Document 2 describes performing vacuum drawing at 1450° C., casting at a temperature of 1360° C., heating and holding at 1100° C. for 6 hours, and subsequently performing furnace cooling. Specifically, first time: heating at 1100° C. for 60 minutes, and thereafter rolling at 2 mm/pass, second time onward: heating at 1100° C. for 30 minutes, and thereafter rolling up to 5 to 7 mm for each pass.
Nevertheless, Patent Document 2 does not in any way describe the problem of cracks in the B-rich phase, or the relation between the size and cracks of the B-rich phase.
Patent Document 3 discloses a Co—Cr—Pt—C-based target, wherein carbides having an average crystal grain size of 50 μm or less exist in the matrix, and the carbides existing in the structure are dispersed when viewing the cross section of the target.
The object of this technology is to prevent the generation of coarse crystals and thereby suppress variations in the characteristics of the magnetic film. A Co—Cr—Pt—C-based material with large amounts of carbides containing C in an amount of 1 at % or more can be subject to hot rolling, enables the refinement of the crystal grain size and the dispersion of carbides, and suppresses variations in the film characteristics such as the coercive force.
Nevertheless, Patent Document 3 does not describe the problem of cracks in the B-rich phase and the solution thereof.
Patent Document 4 and Patent Document 5 respectively disclose Co—Cr—Pt—B—X1-X2-X3 and Co—Cr—Pt—B—Au—X1-X2. While Patent Document 4 and Patent Document 5 offer some description of attempting to improve the brittleness of B based on additives, the method is not very clear. Patent Document 4 and Patent Document 5 merely propose the foregoing compositions, and do not disclose a specific production method. Moreover, Patent Document 4 and Patent Document 5 do not describe the problem of cracks in the B-rich phase and the solution thereof.
Patent Document 6 discloses a sputtering target having a fine and uniform structure by improving the casting process and improving the rolling process of a Co—Cr—Pt—B-based alloy.
As the specific processes to be performed after casting, the ingot is subject to hot rolling at a rolling reduction of 1.33% and temperature of 1100° C. for each pass, and rolling is performed 48 times for causing the crystal grain size of the alloy to be 100 μm or less. The rolling rate in the foregoing case is 55% (rolling rate is roughly 45% to 65%). Nevertheless, Patent Document 6 does not describe the problem of cracks in the B-rich phase and the solution thereof.
Patent Document 7 describes producing a Cr—B target member as a sintered compact. Moreover, Patent Document 7 describes that, since the hard and brittle Cr—B intermetallic compound cracks easily and cannot be subject to plastic working, it is possible to produce a high density target, which is desirable as a target member, by producing the target member as a sintered compact. Nevertheless, Patent Document 7 does not describe the problem of cracks in the B-rich phase and the solution thereof.
Patent Document 8 discloses a Co—Cr—Pt—B-based alloy sputtering target comprising an island-shaped rolled structure made from a Co-rich phase based on the primary crystals during casting, wherein the hot rolling rate is set to 15% to 40%. Moreover, Patent Document 8 describes that, when the hot rolling rate is less than 15%, the dendritic structure cannot be destroyed, and segregation and residual stress cannot be sufficiently eliminated, and when the hot rolling rate exceeds 40%, the Co-rich phase and the B-rich phase become coarse when the processes of rolling and heat treatment are repeated. Nevertheless, Patent Document 8 does not describe the problem of cracks in the B-rich phase and the solution thereof.
Patent Document 9 describes a sputtering target containing B obtained via melting and casting, wherein the B content is 10 at % to 50 at %, remainder is one type among Co, Fe, and Ni, and the oxygen content is 100 wtppm or less. The object of this technology is to achieve a higher density in comparison to conventional powder sintered compact targets by considerably reducing the oxygen content. Nevertheless, Patent Document 9 does not describe the problem of cracks in the B-rich phase and the solution thereof.